1. Technical Field
The present invention relates to motors for driving storage disks and, in particular, pertains to a motor provided with hydrodynamic bearings in which distribution of lubricating oil for the bearings is regulated.
2. Description of Related Art
It is known to furnish motors with hydrodynamic bearing mechanisms for rotatably supporting the rotor in the motor. Such hydrodynamic bearing mechanisms include a pair of thrust dynamic-pressure bearings and a pair of radial dynamic-pressure bearings. A pair of thrust plates along with a sleeve in the motor composes the thrust dynamic bearings. Lubricating oil is retained in the thrust dynamic bearings in a gap between the axially inward surface of each thrust plate and each of axially outward surfaces of the sleeve adjacent to the thrust plate inward surfaces. Striations for generating dynamic pressure in the lubricating oil are formed on either the axially inward surfaces of the thrust plates, the outward surfaces of the sleeve, or both. The substantially cylindrical inner surface of the sleeve surrounds the motor shaft to form a gap between the sleeve and the outer circumferential surface of the shaft radially opposing the sleeve inner surface. Lubricating oil is retained in the gap, and striations for generating dynamic pressure in the lubricating oil are formed on either of the opposing sleeve inner and shaft outer surfaces, or both, to compose the radial dynamic bearings.
Japanese Laid-open patent application No. 09-217735 discloses a motor furnished with a conventional hydrodynamic bearing mechanist including a pair of upper and lower thrust dynamic-pressure bearings, and a pair of upper and lower radial dynamic-pressure bearings. Thrust plates secured on either end of the motor shaft form a pair whose axially inward surfaces together with respectively adjacent axially outward surfaces of the sleeve in the motor form the pair of upper and lower thrust dynamic bearings. The pair of upper and lower radial dynamic bearings is formed by portions of the shaft outer circumferential surface and the sleeve inner circumferential surface, respectively adjacent the upper and lower thrust bearings. The radial bearings are axially separated by an air-filled space for letting air bubbles generated in the lubricant go out of the bearing mechanism. Thus this conventional hydrodynamic bearing mechanism has an upper bearing section, composed of the upper thrust and the upper radial dynamic-pressure bearings, and a lower bearing section, composed of the lower thrust and the lower radial dynamic-pressure bearings. The upper bearing section retains lubricating oil that is continuous throughout the upper thrust and radial dynamic-pressure bearings. Likewise, the lower bearing section retains lubricating oil that is continuous throughout the lower thrust and radial dynamic-pressure bearings. Accordingly, once the motor has been assembled the lubricating oil retained in the upper bearing section is completely separated by the air-filled space from the lubricating oil retained in the lower bearing section.
In the foregoing conventional motor having the conventional hydrodynamic pressure bearing mechanism, the particular configuration of the dynamic-pressure-generating striations in the thrust and radial bearings, together with taper seals formed adjacent the upper and lower bearing sections, prevents the lubricating oil from leaking outside the bearings. The upper and lower thrust bearing gaps taper radially inward to form the taper seals. The striations for the thrust dynamic-pressure bearings are configured as spiral grooves in a formation to urge the lubricating oil radially inward. The striations for the radial dynamic-pressure bearings are configured as herringbone grooves in an axially asymmetrical formation to urge the lubricating oil axially outward from the radial bearings toward the respectively adjacent thrust bearings.
In the motor furnished with foregoing conventional hydrodynamic pressure bearing mechanism, the upper and lower taper seals serve to retain the lubricating oil within the respective upper and lower bearing sections. However, due to characteristics of the lubricating oil at its molecular level, an oil migration phenomenon occurs in which the lubricating oil tends to spread from the bearing sections to areas where the oil normally is not present: along surfaces of the thrust-bearing thrust plates and of the rotor, and along surfaces of the shaft and sleeve in the radial bearings.
The conditions of the foregoing oil migration phenomenon change depending on the material, surface smoothness and like factors of the members in contact with the lubricating oil, such that the lubricating oil disperses by different amounts in different dynamic pressure bearings. Accordingly, when lubricating oil is retained in amounts completely separated from each other in the upper and in the lower bearing sections once the motor has been assembledxe2x80x94as in the case of the foregoing conventional motorxe2x80x94a difference, and therefore an imbalance, occurs between the upper and lower amounts of lubricating oil retained. In addition, wherein the motor assumes its normal or upright posture with its base plate underneath, gravity acting on the lubricating oil in the upper bearing section accelerates the oil migration phenomenon, which increases leakage and dispersion of the lubricating oil.
Moreover, while the motor is rotating, the lubricating oil retained in the upper dynamic bearing section is displaced toward the upper thrust dynamic-pressure bearing under centrifugal force, and by a pumping action created by the axially asymmetric herringbone grooves of the upper radial dynamic-pressure bearing. Consequently, the lower boundary surface of the lubricating oil in the upper radial dynamic-pressure bearing is displaced upward, which exposes the lower ends of the herringbone grooves formed on the shaft or sleeve to the air in the air-filled space. Thus the border of the lubricating oil boundary surface comes to lie on the herringbone grooves, such that the border rises and falls along the corrugated contour of the grooves, which sets up vibrations during rotation of the motor. Motor vibrations thus caused, as well as external vibrations or impact on the motor, dislodge lubricating oil retained in the upper radial dynamic-pressure bearing, such that it drips down into the lower radial dynamic-pressure bearing. This increases the difference in the amount of lubricating oil retained in the upper and lower bearing sections.
When an imbalance occurs between the amount of lubricating oil retained in the upper and lower bearing sections, the dynamic-pressure-generating striations in the bearing that retains the lesser amount of lubricating oil are partially exposed to air, reducing the dynamic pressure produced therein. Consequently bearing rigidity between the upper and lower bearing sections differs, which tends to destabilize the motor rotation. Further, durability and reliability of the motor are impaired by early depletion of lubricating oil from the bearings due to such causes as oil migration leakage.
In the conventional motor, it has in fact been impossible to redistribute the lubricating oil between the upper and lower bearing sections to equalize the retained amounts when there is a difference between the upper and lower sections, since the pair of radial bearings is completely separated by the air-filled space.
Also to be noted is that the above-described imbalance occurs in the amounts of lubricating oil retained in the upper and lower dynamic-pressure bearing sections due to errors in the operation of injecting lubricating oil into the bearing sections.
An object of the present invention is to increase the usable life span of hydrodynamic-bearing-equipped disk-drive motors.
Another object is to improve the reliability and endurance of such disk-drive motors.
A still further object of the invention is, in disk-drive motors equipped with hydrodynamic bearings having bearing sections separated by an air intervention, to equalize the amount of lubricant held in the bearing sections and to take up and re-circulate again to the bearing sections lubricant leaking out of the radial dynamic-pressure bearing portions, in order to maximize rotation stability toward increasing usable life span.
A motor of the present invention is furnished with two hydrodynamic bearing sections separated with respect to the shaft by an air intervention, each composed of a radial and a thrust dynamic-pressure bearing portion, and in each of which lubricant is retained continuously throughout the radial and thrust bearing portions.
The boundaries of the lubricant retained in the two bearing sections front on air. The lubricant boundaries are meniscuses formed by the balancing of the energy of the lubricant itself (the surface tension and intermolecular forces of the lubricant) and of external energy (air pressure of the air contacting the lubricant boundaries and the surface energy of the motor bearing components).
At least one communicating pathway is formed in the sleeve, axially communicating the thrust faces that are constituents of the thrust dynamic-pressure bearing portions of the two bearing sections, the communicating pathway retains lubricant continuously with the pair of thrust bearing portions.
Via the communicating pathway, the lubricant retained in the two hydrodynamic bearing sections shifts mutually from the one section to the other, such that the radii of curvature of the meniscuses forming the respective boundaries are equalized.
A quantitative imbalance occurring between the amounts of lubricant retained in the two bearing sections, for example, when lubricant dislodged from one of the radial dynamic-pressure bearing portions is displaced to the other, means that a discrepancy in the radii of curvature of the meniscuses will arise. When this happens, a discrepancy by the pressure difference between energy of the lubricant itself and the external energy occurs in the lubricant retained in the two bearing sections The discrepancy from the pressure difference sets up an imbalance in the internal pressure of the lubricant retained in the two bearing sections. Due to this internal pressure imbalance, however, a pressure shift occurs via the communicating pathway from that bearing section in which the amount of lubricant retained is greater to that in which the amount is lesser. Consequently, redistribution that equalizes the retained amounts of lubricant in the two hydrodynamic bearing sections takes place through the communicating pathway, eliminating the quantitative imbalance in the lubricant retained in the two bearing sections.
From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art.